Wood provides the most common source of cellulosic material for the manufacture of paper. The wood is transformed into a paper pulp suspension using either a mechanical process in which the fibers are broken down through physical contact with, for instance, a rotating member or a chemical process, for instance, involving pressure-cooking wood chips in a suitable chemical solution.
The chemical process is preferred as it provides not only a pulp having an average fiber length longer than that produced by the mechanical process but also a stronger fiber, both of which factors lead to a better quality paper. After pulping of the wood it is "refined", a process which frays and flattens the pulp fibers making the fibers softer, more plastic and increasing their surface area. These conditions lead to the fibers bonding more strongly to one another to produce a stronger and therefore better quality paper. Sizing materials, mineral pigments, etc. may then be added to the pulp suspension.
At this stage, the pulp suspension may have a solids content of only one percent and generally has a thin milky appearance.
Where top quality paper is required, for instance for important documents, the paper is "hand made" from the pulp. The pulp suspension, contained in a suitable tank, is bottom-stirred to keep it in a uniform suspension. A sieve is then lowered, edge first, into the tank and the edge is then drawn slowly towards the papermaker until the sieve occupies a substantially horizontal position. As a result, pulp is collected on the sieve.
The sieve is then raised and the water allowed to drain away. By means of various canting and shaking movements, the papermaker causes the pulp to settle in a uniform mat which is further processed to provide sheets of paper.
A characteristic of paper made as described above is that the fibers are arranged in all directions without there being noticeable orientation of fibers in a particular direction. As a result, a paper of top quality is produced which has substantially the same parting strength in all directions contained in the plane of the paper.
One of the earliest machines developed for the industrial manufacture of paper is the so called Fourdrinier machine. In this machine there is employed an endless screen in the form of a mesh belt. The fibers, fillers, pigments and the like in the suspension are allowed to flow from a head box through below a "slice" or gate onto the belt in a direction substantially parallel to the horizontal surface of the belt. Water, together with "fines" comprising short-length fibers, filler, pigment and other fine materials pass through the belt until the web is fairly well packed on the belt after which stage there is a tendency for the "fines" to be trapped in or on the web.
The web is then further processed to remove excess water by passing the belt and web along suitable rollers and suction boxes. The web may then be lifted off the belt and further treated to provide the desired paper product.
When the pulp passes through below the "slice" or gate it is travelling at approximately the same rate as the belt. As the fibers flow from under the "slice," there is a tendency for them to line up in the direction of travel of the belt. In the art, such direction of travel is commonly referred to as the "machine direction". As a consequence of such fiber orientation, paper made on a Fourdrinier-type machine has greater strength and stretchability in the machine direction that in a direction transverse thereto. This disparity between the strength and stretchability of machine-made paper in the machine and cross-machine direction increases with increase in the speed of the screen until, at high speeds, only a poor quality paper can be produced.
In an attempt to equalize more nearly the strengths of the finished paper in the machine direction and in the cross-machine direction, the screen support was made to oscillate horizontally, thus providing a shake in the screen as the water is drained from the web. For both economic and mechanical reasons it has been found impractical to provide an oscillating screen at screen speeds above 1,000 feet per minute and the use of such oscillating mechanisms must, therefore, be abandoned in the high speed production of pulp webs for paper and board production.
An object of the present invention is the provision of a method and apparatus for use in the high speed manufacture of paper and board in which the degree of fiber orientation during laying-up of the pulp web is reduced when compared to the degree of fiber orientation obtained on a Fourdrinier-type machine at similar high speeds.